Many devices that employ magnetic fields have heretofore been encumbered by massive solenoids with their equally bulky power supplies. Thus, there has been increasing interest in the application of permanent magnet structures for such uses as electron-beam focusing and biasing fields. The current demand for compact, strong, static magnetic field sources that require no electric power supplies has created needs for permanent magnet structures of unusual form. A number of configurations have been designed and developed for electron beam guidance in millimeterwave microwave tubes of various types; for dc biasing fields in millimeter wave filters, circulators, isolators, striplines; for field sources in NMR (nuclear magnetic resonance) imagers; and so on. Especially promising for such purposes is the configuration based upon the hollow cylindrical flux source (HCFS) principle described by K. Halbach in "Proceedings of the Eighth International Workshop on Rare Earth Cobalt Permanent Magnets", Univ. of Dayton, Dayton, Oh., 1985 (pp. 123-136). A HCFS, sometimes called a "magic ring", is a cylindrical permanent magnet shell which produces an internal magnetic field that is more or less constant in magnitude. The field is perpendicular to the axis of the cylinder, and furthermore the field strength can be greater than the remanence of the magnetic material from which the ring is made.
The ideal magic ring is an infinitely long, annular cylindrical shell which produces an intense magnetic field in its interior working space. The direction of the magnetic field in the working space interior is perpendicular to the long axis of the cylinder. The aforementioned Halback publication discloses a structure with an octagonal cross section which closely approximates the performance and field configuration of an ideal magic ring (which has a circular cross section). In both the ideal and Halbach configurations, no magnetic flux extends to the exterior of the ring structure (except at the ends of a finite cylinder).
The term "magic ring" as used herein encompasses not only the ideal cylindrical structure but also octagonal, sixteen sided, thirty-two sided and even higher order polygonal-sided structures which approximate the ideal magic ring.
Unfortunately, the magic ring is theoretically infinitely long. Thus, achievement of the desirable high uniform magnetic fields in the interior of the magic ring structure demands that the structure be made extremely long (theoretically infinite). If the structure is not long enough, distortion of the interior fields will result.
Those concerned with the development of high power microwave devices such as wigglers and twisters have continually searched for means to create intense magnetic fields in confined spaces with lightweight devices.
A wiggler is a high power (megawatt) radiation source. In wiggler operation, an electron beam is injected into a drift region which is surrounded by a periodic permanent magnet source. The periodic permanent magnet source creates a magnetic field which changes directions (by 180.degree.) at fixed intervals, yet is always perpendicular to the principal direction of electron beam travel. (By contrast, and an twister is an electron beam device in which the magnetic field orientation changes more gradually).
The advent of magnetically "hard" materials, i.e. magnetic materials which maintain their full magnetization against fields larger than their coercivities, permits the fabrication of many novel magnetic structures. Examples of magnetically "hard" materials are: neodymium iron boride (Nd.sub.2 Fe.sub.14 B), samarium cobalt (SmCo.sub.5), platinum cobalt, (PtCo.sub.5), and a samarium cobalt alloy, (Sm.sub.2 (CoT).sub.17 , where T represents one or more transition metals) together with selected ferrites. These materials may be pressed or fabricated into various desired shapes and magnetized in a variety of desired orientations using techniques known to those skilled in the art.